Physical development process utilizing viscous sensitizing metal solution

ABSTRACT

A NEW PROCESS IS DESCRIBED FOR PRODUCING IMPROVED METAL IMAGES IN PHOTO-EXPOSED MEDIA COMPRISING A PHOTOCONDUCTOR AS THE PHOTOSENSITIVE COMPONENT INCLUDING THE STEP OF CONTACTING THE MEDIUM WITH A VISCOUS REAGENT COMPRISING A SENSITIZING METAL ION, E.G. SILVER ION.

United States Patent 3,829,317 PHYSICAL DEVELOPMENT PROCESS UTILIZING VISCOUS SENSITIZING METAL SOLUTION Laura K. Case, Winchester, Mass., assignor to Itek Corporation, Lexington, Mass.

No Drawing. Continuation of abandoned application Ser. No. 812,378, Apr. 1, 1969. This application Aug. 3, 1972, Ser. No. 277,703

Int. Cl. G03c 5/24, 5/32 US. C]. 96-48 PD 19 Claims ABSTRACT OF THE DISCLOSURE A new process is described for producing improved metal images in photo-exposed media comprising a photoconductor as the photosensitive component including the step of contacting the medium with a viscous reagent comprising a sensitizing metal ion, e.g. silver ion.

This is a continuation of application Ser. No. 812,378, filed Apr. 1, 1969, now abandoned.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to development of photographic images and more particularly photographic images in media in which the photosensitive component is a photoconductor.

Description of the prior art 'Photosensitive media comprising radiation sensitive materials such as titanium dioxide are described in detail in US. Pats. 3,152,093; 3,052,541; French Pats. 345,206 and 1,245,215 and in British Specification 1,043,250. In the aforementioned British Specification, radiation-sensitive titanium dioxide functions as a photosensitive component of the media and exposure of said media to activating means such as radiant energy, electron beams or the like results in the storage of a reversible latent image patten therein. The reversible latent image pattern exists for a finite time during which said pattern can be converted to an irreversible form and read out visually by contacting said pattern with a suitable image forming material, such as a chemical redox system. In the aforesaid US. and French Patents, the radiation-sensitive material is combined with at least one component of an image-forming material prior to exposure to activating means. On the other hand, US. Pat. 3,152,903 discloses a system wherein the photosensitive material is used in combination with both an oxidizing agent such as silver nitrate and a reducing agent such as hydroquinone. Upon exposure to suitable activating means, a visible image is formed.

One of the features of the above-mentioned data or image storage systems is that the photosensitive materials are often sensitive to a very narrow range of electromagnetic radiation. Therefore, it is often desirable to sensitize these photosensitive materials to additional ranges of electromagnetic radiation by uniformly exposing the photoconductor to bandgap light followed by decay of the resulting activation which then renders the decayed photoconductor sensitive to light of a different Wavelength, usually light of a longer wavelength extending into the near visible, to the visible region and in some cases to the infrared region. Such sensitization of photoconductors is described in commonly assigned copending US. application Ser. No. 653,198 filed July 13, 1967.

'One of the problems of obtaining visible metal images of good optical density resides in the manner of development selected. For example, when the exposed medium is first immersed in a solution of sensitizing metal ion, e.g. silver ion, and then into a solution of reducing agent, e.g. hydroquinone, the optical density, while acceptable, is not always suitable for photographic use and intensification of the visible image is often required. As an alternative, it is possible to contact the exposed medium to a single medium comprising both sensitizing metal ion and the reducing agent. Such single media are suitable for single development in that acceptable optical densities are generally obtained but are undesirable for multi ple development or continuous use since there is considerable loss of sensitizing metal due to reduction on long term exposure to the reducing agent, and further, because of contamination of the single developing medium, undesirable effects, e.g. scum formation, in the developed photographic image can occur. Thus such single media are extremely wasteful in terms of uneconomical loss of silver ion due to the fact that they require frequent changing and do not lend themselves to automatic replenishment systems commonly employed in high speed photographic developing.

US. Pat. 3,372,029 describes the development of photographic images simultaneously on a first medium comprising zinc oxide as the photosensitive component on a conductive backing and a second medium comprising a photosensitive silver halide by use of a thickened developer solution of a reducing agent together with a solvent for the silver halide, i.e. a soluble thiosulfate, but only while the developer solution is in conductive contact with the conductive backing of the zinc oxide medium. In effect, the source of silver in formation of the visible, i.e. silver, image is the silver halide which is dissolved in the developer solution by action of the thiosulfate to form a weakly dissociated, negatively charged complex of silver. The apparently poor quality of the visible images obtained on the zinc oxide medium, i.e. in the words of the patentee, a weak, visible image, indicates that the process is not effective from the viewpoint of directly producing a practical visible image unless a further step of image intensification is employed. Further, as indicated by the patentee, the process is totally inoperable when conductive contact is not maintained between the developer solution and the conductive backing of the zinc oxide medium.

DESCRIPTION OF THE INVENTION It has now been surprisingly found that improved metal images are obtained when a photo-exposed medium comprising a photoconductor is contacted with a viscous reagent comprising a sensitizing metal ion. The optical density of the image is substantially greater than that obtained by development with separate solutions of sensitizing metal ion and developer respectively, and is at least of the same order of magnitude as that obtained using a combined liquid developer system which includes both sensitizing metal and reducing agent, but without the disadvantages of the latter.

In general, the photoexposed medium is coated with a viscous layer of the sensitizing metal and then is contacted with the reducing agent. As a matter of convenience, the

reducing agent can be incorporated into the viscous layer of sensitizing metal or applied separately, for example, as a second viscous layer or, alternatively, in a suitable solution into which the medium coated with the viscous layer can be immersed.

DESCRIPTION OF PREFERRED EMBODIMENTS and. tin 'disulfide 1( SnS and metal selenides such as cadmium selenide (CdSe). Metal oxides are .espec1ally..-

preferred photoconductors of this group. Titanium dioxide is a preferred metal oxide because of its unexpectedly good results. Titanium dioxide having an average particle size less than about 250 millimicrons and which has been treated in a reducing atmosphere at a temperature between about .200 C- and 950 C. for about 0.5 hours to about 30 hours. and then rapidly quenched is especially preferred, and. more especially that titanium dioxide produced by high temperature pyrolysis of titanium halide.

Also useful'n this invention as photoconductors are certain fluorescent materials. Such materials include, for example, compounds such as silver activated zinc sulfide and zinc activated zinc oxide.

While the exact mechanism by which this invention works is not known, it is believed that exposure of the photoconductors or photocatalysts to the activating means causes an electron or electrons to be transferred from the valence band of the photoconductor or photocatalyst to the conductance band of the same or at least to some similar exicted state whereby the electron is loosely held, thereby changing the photoconductor from an inactive form to an active form. If the active form of the photoconductor or photocatalyst is in the presence of an electron accepting compound a transfer of electrons will take place between the photoconductor and the electron accepting compound, thereby reducing the electron accepting compound. Therefore a simple test which may be used to determine whether or not materials have a photoconductometric or photocatalytic effect is to mix the material in question with an aqueous solution of silver nitrate. Little, if any, reaction should take place in the absence of light. The mixture is then subjected to light, at the same time that a control sample of an aqueous solution of silver nitrate alone is subjected to light, such as ultraviolet light. If the mixture darkens faster than the control, the material is a photoconductor or photocatalyst.

It is evident that the gap between the valence and the conducting band of a compound determines the energy needed to make electron transitions and the light required to provide the needed energy is called bandgap light, as employed herein. The higher energy needed, the higher the frequency to which the photoconductor will respond. It is known in the art that electrons may be present in secondary levels within the bandgap due to impurities or defects in the structure of the photoconductor. With light of suitable energy, which in this case would be less than the bandgap, electrons from these levels could be raised to the conduction band. A typical example of a secondary level due to the defect in the structure would be an F-center (electrons trapped at negative ion vacancies in an alkali halide crystal). The band gap of KCl is about 8.5 e.v. (1460 A.), but the secondary levels due to F-centers are about 2.4 e.v. (5400 A.) below the conduction band. Electrons could be raised to the conduction band with (5400 A.) light. An example of the effect of an impurity is ZnS doped with Cu. The band gap of ZnS is about 3.7 e.v. (3350 A.), but by doping it with Cu one could introduce some secondary levels'which would result in photoconduction due to 4600 A. light.

'The preferred media for use in this invention are those in'which the photoconductor in a binder is coated on a surface comprised of a base material such as plastics, (e.g.cellulose acetate, nylon or polyester, especiallypolyethylene terephthalate), which may be in the form of sheets, films or ribbons, or alternatively may be coated on-"inert supports, e.g.-wood, paper and the likei g lass metals such as iron and preferably aluminum, and' the' like. 'The' said base materials are those which are unimbibable withthe usual development solvent systems, e'Ig." water and/or alcohols such as. methanol. Especially preferred base materials are those which are transparent,

.r-metalbase materials,especiallyaluminum.

eigf the plastic base materials. Also preferred are the The sensitizing metal isany metal which on contact with photo-activated areas of the photoconductor-bearing layer will be reduced to form a residue which deposits in the photo-activated areas thus rendering the said areas visible or potentiallyvisibleln the latter case, the deposited metal is designated a latent metal image which canbe further treated to obtain a visible image, i.e. by image intensification or amplification, using known methods. Suitable sensitizing metals include those metal ions which are at least as strong oxidizing agents as copper ions, e.g. silver, cupric, cuprous, mercury, gold, platinum, palladium ions. Preferably, the sensitizing metal ion is silver ion. 1

Suitably, the sensitizing metal ion is maintained in the viscous layer at the highest activity which is preferably accomplished by the use of highly dissociable sources of the metal ion.' The most convenient source is simple soluble salts of the metal ion which are highly dissociable to provide high activity of the positive metal ion in solution. For example, as a source of silver ion, silver nitrate is preferably employed since it is readily soluble and highly dissociable.

If, for any reason, it is desired to use complex ions containing the sensitizing, metal ion, stable complex ions must be avoided since the activity of the sensitizing metal ion will not be sufiicient to obtain the desired results attainable by the present invention. Such complex ions as that formed between silver ion and thiosulfate ions are too stable to be operable in the present invention and when they are used only weakly visible images, e.g. those of low optical density are obtained.

For general purposes, the concentration of the sensitizing metal ion in theviscous layer should be at least 1X10 moles per liter to obtain the improved densities attainable with the present new process. The concentration of sensitizing metal ion can be varied considerably depending on the usual considerations known to those skilled in photographic developing. Usually, when the preferred metal ion, silver ion, is used, it is desirable to use the minimum effective concentration thereof in view of the relative cost and availability of silver salts. A minimum of experimentation will permit the selection of suitable concentrations of the sensitizing metal ion for whatever result may be desired.

The binder of the layer of photoconductor can be any of a variety of materials which include those normally employed in photography. Exemplary binders are polyvinyl alcohol, gelatin, polymers and copolymers of acrylarnide, e.g. polyacrylamides, polyalkylacrylates, styrene-butadiene copolymers and polyvinyl acetate.

The viscous layer of sensitizing metal is prepared from a solution contaning the metal ion by addition of a thickening agent. Suitable thickening agents are selected on the basis of usual considerations derived from a knowledge of photographic development. The selected thickening agent should not adversely aifect the desired chemical reaction to any appreciable extent i.e., the thickening agent should be photographically acceptable. For example, the thickening agent should not react with sensitizing metal ion to appreciably reduce the activity thereof in the viscous layer, nor should it react appreciably with the selected reducingagent, i.e. the developer, in an adverse way. Further, the thickening agent should be substantially inert with respect to the photographic medium being developed. 7 k

I The thickening agent can be any of a variety of known substances, including both naturally-occurring and. synthetic materials, For example, various cellulose derivatives" can'be' used including methylcellulose, ethylcellulose, hydroxypropylcellulose, hydroxypropyl methylcellulose, andthe like. Natural thickening agents include gelatin, e.g. calf gelatin or pig gelatin, pectin and carrageenin (Irish moss extract). Synthetic thickening agents are exemplified by hexitol anhydride fatty acid esters such as sorbitan monolaur'ate, sorbitan stearate, and similar esters of the higher'fatty acids as well as the polyoxalkylated derivatives thereof, e.g. polyoxyethylene derived of sorbitan monolaurate. Synthetic polymeric materials are exemplified by polymers and copolymers of acrylamide, among others.

The solvent systems for the sensitizing solution are determined by the nature of the sensitizing metal ion source, the photographic medium, particularly the binder of the photosensitive layer thereof and other considerations known to those skilled in the art. In general, aqueous systems are employed, but in some instances it is desirable to utilize alcohol systems, e.g. lower alkanols, especially methanol, to achieve the desired result with some binder systems, e.g. polyvinyl acetate for which a methanol system is preferred.

The viscosity of the layer should preferably be at least about 500 centipoises and can conveniently be in the range of from about 500 to about 10,000 centipoises and preferably ranges from about 1,000 to about 5,000 centipoises. Suffice it to say that the viscosity of the layer is dependent on the specific application and can be varied widely as is obvious to those skilled in the art, even to assume the form of a gel.

The following examples are given to further illustrate the invention described herein.

EXAMPLE 1 Solutions of silver nitrate are prepared by adding various amounts of methyl cellulose (5% aqueous solution) to a solution of 5g of silver nitrate in 50 ml. H and 50 ml. methanol and the resulting solutions are used in development of an exposed medium consisting of titanium dioxide in a methylmethacrylate polymer binder (Rhoplex HA-16, Rohm & Haas) coated on cellulose acetate sheet.

G Methyl Cellulose Solution) Solution The following solutions are prepared: Solution A 16.2 g. ferric nitrate 39.2 g. ferrous ammonium sulfate 10.5 g. citric acid 250 ml. water Solution B 0.25 g. Armac 0.25 g. Syntbrapol 250 ml. water Solution C 42.5 g. Silver nitrate 100 ml. water.

EXAMPLE 3 A developer is prepared by mixing 100 ml. Solution A, 80 ml. Solution B and 8 ml. Solution C with 20 ml. 7

of water.

A mixture of 4 parts of titanium dioxide and 1 part of a methyl methacrylate polymer (Rohm & Haas, I-LA-8) is coated on a polyethylene terephthalate sheet and the medium exposed to an image using a one second exposure (Besseler Box).

The exposed medium is coated with the developer for a total of 25 seconds and the developer is then washed off with water. Total density=2.0.

When the exposure time is increased to 45 seconds, the total density of the image obtained is 2.2. 1

A viscous solution is prepared by mixing 10 g. of a polyacrylamide (Cyanamer P-250, American Cyanamid), 85 g. of silver nitrate in enough water to make one liter.

A photoexposed planographic plate consisting of a layer of titanium dioxide in polyvinyl alcohol on aluminum sheet is coated with a layer of the viscous solution 1-3 mils in thickness after which a viscous layer of reducing agent (10 g. of Cyanamer P250, g. citric acid, 30 g. Metol-sulfate in one liter of water) is applied (thickness 4-10 mils).

The medium is allowed to stand for from 1-10 minutes and the medium is rinsed with running warm water to remove the viscous layers.

The density obtained is equivalent to monobath solution processing (silver nitrate, citric acid and Metol-sulfate in one solution).

The disadvantage of the monobath solution processing is the fact that the monobath may not be re-used and must be discarded after single usage.

EXAMPLE 4 A brush-grained anodized aluminum sheet coated with a layer of titanium dioxide in polyvinyl alcohol is photoexposed and then processed as follows.

The plate is coated on a Gardner applicator with a. layer (0.006 in.) of silver nitrate (0.5 to 1.5M) containing a commercial thickener (Klucell-Hercules Powder). After 60 seconds, a thickened layer (1 mil) of saturated solution of Metol containing 45 g./l. of citric acid monohydrate is applied. After 60-90 seconds, the sheet is rinsed in tap water to remove the treating layers and unatfected polyvinyl alcohol layer leaving only the silver image.

The silver image is adherently bonded to the aluminum sheet (passed the Scotch Tape Test) and the sheet can be used as a lithographic plate.

When the present process is employed with photoexposed media as described in Examples 3 and 4, a permanent, adherently-bonded silver image is obtained. In addition, the silver image is continuous with respect to the type of silver deposit as contrasted with granular or particulate deposits of silver which latter form is undesirable when the developed medium is to be employed as a planographic plate.

The planographic plates developed in accordance with the present invention are particularly suited for their intended use, i.e. for printing since the silver image is adherently bonded to the aluminum metal substrate through the anodized coating, i.e., is conductively bonded to the aluminum substrate. The conductive bonding is especially desirable in providing a plate of relatively long life, i.e. permits production of a large number of copies from the master. In addition, the conductive bonding permits amplification of the silver image in electroless plating baths such as copper amplification as described in commonly assigned copending U.S. application Ser. No. 743,981, filed July 11, 1968.

The viscous layer is preferably applied to the photosensitive medium subsequent to photoexposure. However, if desired, the viscous layer can be present on the medium at the time of photoexposure with proper precautions. For example, the light selected for photoexposure should not cause reduction of the sensitizing metal in the viscous layer which would lead to reduction of the activity of 5 the ion. Further, the viscous layer should not absorb appreciable amounts of the selected exposure light to reduce the efiiciency of exposure. Other obvious precautions suggest themselves to those skilled in the art and will be determined by many factors for which standard techniques are provided in the art.

What is claimed is:

1. In a process of producing a metal image in a photoexposed imaging medium comprising an inorganic photoconductor which upon exposure to activating radiation becomes chemically reactive in exposed portions of said medium as the photosensitive component thereof which includes the step of contacting said medium with a sensitizing metal ion and contacting in a separate step said medium with a reducing agent for said metal ion, said metal ion being of sufficient concentration and activity to be readily reduced upon contact with the photoexposed, chemically reactive photoconductor, the improvement wherein said metal ion forms part of a viscous reagent having a viscosity of at least about 500 centipoises.

2. Process as in Claim 1 wherein the viscous reagent is of a viscosity of from about 100 to about 5000 centipoises.

3. Process as in Claim 1 wherein the metal ion is silver 4. Process as in Claim 1 wherein the photoconductor is dispersed in a layer of binder on a suitable substrate.

5. Process as in Claim 1 wherein the photoconductor is a compound of a metal with a non-metal of Group VI A of the Periodic Table.

6. Process as in Claim 1 wherein the photoconductor is a metal oxide or sulfide.

7. Process as in Claim 2 wherein the photoconductor is titanium dioxide.

8. Process as in Claim 7 wherein the titanium dioxide is of an average particle size of about 250 millimicrons or less.

9. In a process of producing a visible metal image in a photo-exposed imaging medium comprising an inorganic photoconductor which upon exposure to activating radiation becomes chemically reactive in exposed portions of said medium as the photosensitive component thereof which comprises contacting said medium with (A) a sensitizing metal ion, said metal ion being in a concentration and of such activity to be readily reduced upon contact with the photo-exposed, chemically reactive photoconductor; and then in a separate step (B) a reducing agent for said metal ion, the improvement wherein said metal ion is in the form of a viscous reagent having a viscosity of at least about 500 centipoises; and wherein said metal ion is in solution in said reagent in a concentration of at least 1 10 moles per liter of solution.

10. Process as in Claim 9 wherein the reducing agent is provided in a liquid form in a separate step.

:8 I 11. Process as in Claim 9qwherein thereducing agentis provided in the-form .of a viscous layer; and wherein the viscous layer is of ,a viscosity of at,-least, about- 500 centipoises. j' s 12. In a pro ess for producing a metal-image in a photo-exposed imaging medium. which. is comprised of titanium dioxide which upon exposure'to'activating radiation becomes-chemically reactive in exposed-portions of said medium ina binder therefor on a suitable substrate which process comprises the step of I contacting said medium with a sensitizing metal ion and in a separate step a reducing agent for said metal ion, said metalion being of suflicient concentration and activity .tohe readily reduced upon contact with the photo-exposed .chemically reactive titanium dioxide photoconductor, the improvementcorn prising incorporating said metal ion in solution witlra thickening agent to form a viscous reagent having aviscosity of at least about 500 centipoises. g v I 13. Process as in. Claim: 12 'whereinl the binder is a transparent binder. '7

14. Process as in Claim transparent substrate. I

15. Process as in Claim 14 wherein the said metal ion is at least as strong an oxidizing agent as cupric ion.

16. Process as in Claim 14 wherein the said metal ion is silver ion. v

17. Process as in Claim 12. wherein the substrate is comprised of aluminum sheet, polyethylene terephthalate or cellulose acetate. 7

18. Process as in Claim 12 wherein the binder is polyvinyl alcohol, gelatin, a polyacrylamide, polyvinyl acetate, a polyalkyl acrylate or a styrene-butadiene polymer.

19. Process as in Claim '1 wherein the metal ion is palladous ion.

13 wherein the substrateis a References Cited UNITED STATES PATENTS 3,380,823 4/1968 Gold 96-48 3,382,068 5/1968 Gold 9648 3,152,903 10/1964, Shepard et al 96-64 3,372,029 3/1968 Nail 96-88 3,623,868 11/1971 .Cronig 9648 PD FOREIGN PATENTS 9/1966 Great Britain 9648 RONALD H. SMITH, Primary Examiner W. H. LOUIE, JR., Assistant Examiner US. or. .X.R. 9660 R 

